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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .tris{
            width: 0;
            height: 0;
            border: 20px solid transparent;
            border-top: 20px solid blue;
        }
    </style>
</head>
<body>

    <div class="tris"></div>

    <div style="width: 200px; height: 100px;transform: rotate(45deg) translateX(-100px);">

        <div style="float: left;width: 100%;height: 100%; background-color: red;">3</div>
    </div>
    
</body>
</html>